Summary The Israel Aircraft Industries Astra SPX aircraft (registration C-GSSS, serial number80), with two crew and four passengers on board, was landed on Runway05 at Toronto Pearson International Airport at 1826 eastern daylight time. As the nosewheel touched down, a severe nosewheel shimmy developed, and the flight crew had difficulty controlling the aircraft. As the flight crew attempted to steer the aircraft, an uncommanded full-left steering input was experienced, and the aircraft began to veer to the left. The first officer attempted to turn the steering control to the right, but was unable to move the control. The flight crew attempted to correct for the full-left input using differential braking and reverse thrust, but were unable to keep the aircraft on the runway. The aircraft skidded off the north side of the runway and came to rest in the infield between Runway05 and taxiway Juliet, just before the intersection at Runway15R. The captain contacted the tower and requested emergency services. Meanwhile, the first officer exited the aircraft to check for damage and to ensure there was no further danger to crew or passengers. Assessing the situation to be safe, the first officer re-entered the aircraft, and the flight crew and passengers waited for emergency services to arrive. There was minor damage to the aircraft. Ce rapport est galement disponible en franais. Other Factual Information The captain occupied the right seat, held a valid airline transport pilot licence, and had recently completed a pilot proficiency check on type. He had accumulated a total flying time of approximately 17500hours, with 500hours on type. The first officer held a valid airline transport pilot licence and had recently completed the Astra/G100 pilot recurrent course. He had accumulated a total flying time of approximately 8300hours, with 775hours on type. At the time of the occurrence, the first officer was the pilot flying, occupying the left seat. This allowed the first officer to cover all aspects of his flight leg, including taxiing, since the AstraSPX is equipped with only one steering tiller located on the left side of the cockpit. The first officer was qualified to occupy this seat. The 1800 eastern daylight time1 weather report for Toronto Pearson International Airport was as follows: wind 100degrees True at 8knots, gusting to 15knots; scattered clouds at 1500 feet above sea level (asl); broken cloud at 2800feet asl; visibility greater than 15miles; temperature 17C; dew point 10C. The runway was bare and dry. Magnetic variation at Toronto is 10west. Examination of the aircraft revealed that the nose landing gear steering assembly upper and lower attachment brackets had failed near the attachment to the aircraft bulkhead. The left steering actuator was found fully extended and the right steering actuator was found fully retracted; this corresponds to a full-left steering input. The entire steering assembly was rotated and jammed against the right side of the nose landing gear bay, beyond the normal steering range for the nose landing gear. Photo1. Rear view of steering assembly removed from the aircraft Photo2. Upper and lower attachments on aft bulkhead. The steering assembly (seePhoto1) comprises an upper and lower attachment bracket, a left and right steering actuator, and a shaft with a universal joint to mate to the nose landing gear. The brackets are bolted together at four points using nuts and bolts with a spacer to maintain separation. The entire steering assembly is then bolted to the aircraft at the nose landing gear bay aft bulkhead. There was fretting2 found on the mating surfaces of the upper and lower brackets at the attachment to the aircraft bulkhead (seePhoto2). The upper bracket fracture surfaces were heavily contaminated with blackened Skydrol hydraulic fluid, and had a polished appearance. As well, the paint around the fracture surfaces was softened and blistered in much the same manner as a painted surface with paint stripper applied. By comparison, the lower bracket fracture surfaces were clean and dull in appearance, and the paint showed no signs of softening or blistering. Both the upper and lower bracket fracture surfaces had smearing damage from the fracture surfaces impacting each other. The entire steering assembly was forwarded to the Transportation Safety Board (TSB) Engineering Branch Laboratory for further examination. Laboratory examination showed there was no indication of fatigue cracking on any of the fractured surfaces and all of the fracture surfaces were representative of overstress failures. A test was conducted to see how long it would take for the paint to start blistering if contaminated with Skydrol hydraulic fluid. This consisted of scratching through the paint to the metal on a cleaned portion of the attachment bracket. This would be representative of a typical crack in the paint at a fracture site. The area was then contaminated with Skydrol hydraulic fluid; it took approximately three weeks for the test area to show any signs of paint blistering. The aircraft was maintained on an approved phase inspection program, with inspection packages due every 62.5flight hours. The steering actuator assembly, including attachments, is inspected every 250hours or every fourth phase. The last time the steering assembly was inspected was during the Phase5 inspection, 171hours prior to the occurrence on 25April2003. The inspection revealed no discrepancies. An examination of the aircraft logbooks showed several instances of nosewheel shimmy dating back to 23March2002. Most of these were directly attributed to a loose Bnut fitting on the steering selector valve. When steering is not being actuated, hydraulic fluid flows through restrictors in the steering control valve, dampening nosewheel shimmy. The loose Bnut allowed hydraulic fluid to leak excessively and hinder the dampening characteristics of the steering selector valve. Tightening of the Bnut eliminated the problem in all but one case, which was attributed to a loose nosewheel torsion shaft. There is no means of ensuring the safety of the Bnut to prevent it from loosening. Worldwide, there have been six other occurrences of nosewheel steering assembly bracket failures on this type of aircraft. This is the first failure on a Canadian-registered aircraft and the first TSB investigation into this type of failure. In most cases, a nosewheel shimmy was experienced by the flight crew prior to the failure being discovered. An examination of these failures indicated they were the result of exceeding the steering limits with the steering links (scissors) connected. The aircraft manual stipulates the maximum steering range for the nose landing gear is 58either side of centre. It also cautions that, The nose landing gear can be damaged if turn limits are exceeded with scissors connected or due to low strut. The maximum steering angle with the scissors disconnected is 90either side of centre. There are no visual indicators of the maximum steering ranges. The operator has indicated that they try to ensure the aircraft is towed with the scissors disconnected at all times. However, this is not always possible. The aircraft is often parked away from home base, at a fixed base operator, and, due to operational requirements, the aircraft may be moved to an alternate parking position without the flight crew's knowledge. There have been several service bulletins (SBs) issued that relate directly to the nose gear steering system on AstraSPX. On 25June1998, the manufacturer issued SB1125-11-181, which added the warning placard DISCONNECT SCISSORS BEFORE TOWING to the landing gear doors. The warning placard serves as a visual reminder for ground-handling personnel to disconnect the scissors prior to towing, to prevent damage to the nose landing gear steering brackets. This SB was further emphasized on 25June1999, through service information letter No.1125-09-096, recommending a review of the towing procedures in the Astra maintenance manual, Chapter9-00-00 (page1) and Chapter9-10-00 (pages1through6), and the incorporation of SB1125-11-181. On 26 February2001, the manufacturer issued SB1125-32-225, which provided instructions to modify the nosewheel steering angle limitation system, in order to reduce the maximum steering angle by up to 2. This would eliminate hydraulic hammer3 at the end of the steering stroke while using extreme steering angles during taxiing. In addition, on 20April2001, the manufacturer issued SB1125-32-187, which called for the replacement of the bell crank connecting shaft on the steering assembly. This replacement was intended to prevent over tightening of the bell crank connecting nut, which could cause stiff nosewheel steering and might affect the centring of the nose gear. None of the SBs noted above were complied with prior to the occurrence, nor was compliance required by regulation. Transport Canada issued Airworthiness Notice (AN)B055 Edition1, 4July2000, titled Service Bulletin Compliance, clarifying the need to comply with service bulletins. Section2 of the ANdeals with publications that recommend the incorporation of modifications, the performance of inspections, or times between overhaul. This section states, in part, the following: Except where otherwise specified in Std625, AppendixC, compliance with publications dealing with these topics is optional. However, aircraft owners have a duty to be aware of the contents of these publications, and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation, as part of the evaluation program required by CAR4706. As such, the SBs were reviewed by the person responsible for maintenance at the time they were issued, and a decision was made not to implement them.